Technical Info from FAS part 4

[kuya jhas]

Cylinder volume - if the cylinder does not have the internal volume to hold enough air to propel a BB down a certain barrel length, the BB will slow down in the barrel due to not longer being accelerated by a force behind it (air pressure). Conversely, if it has more internal volume than the inner barrel, the BB will not reach full velocity before exiting the barrel because the initial rush of air is what propels the BB, not the full volume being expelled from the cylinder by the piston racing forward. An example would be Obsidian’s G3SAS: It has a very short inner barrel with a type0 cylinder if I recall correctly, so the BB is not reaching the full acceleration before exiting the barrel, so thus he installed a stronger spring to compensate. In other words, he is using a spring that is capable of more energy with parts that stunt that capability, thus allowing that particular gun to be legal. A different approach would be to use the appropriate cylinder for that short of a inner barrel length, which would allow that gun to reach about the same muzzle velocity with a weaker spring, thus saving some wear and tear on other mechbox internals. In those two scenarios, the gun has the same muzzle energy, but went two different routes to get there, with one being optimized for that barrel length and the other using brute force.

Thus, this is why various cylinder types (positioning of holes in the cylinder) are matched with certain barrel lengths, because you want as much air moving as fast as you can get it to be what is expelled through the nozzle.

When an AEG fires, it is expelling a set amount of air. Since the air from the cylinder is at the same pressure as the atmosphere, the BB is only accelerated by the volume of air behind it. This differs from a gas gun, in which a certain amount of gas is expelled or valve simply opened, which allows the propellant gas to expand rapidly and try to equalize with the atmosphere. In that case, the gas will actually cause the BB to gain velocity in the barrel as the gas increases speed in order to escape the barrel to equalize with the atmosphere.

Now in both cases, BB weight can cause various muzzle velocities without changing the spring. This is because a higher-powered gun is forcing air out that much faster. Thus, a light BB might be expelled very quickly by that initial burst and not reach the energy that the gun is actually capable of......in other words, if a heavier BB is used in that same gun, you can actually see a muzzle energy increase because the BB is heavy enough to not be shot forward by that initial burst. This means the full volume of air is behind it moving quickly, and it reaches full acceleration as it exits the barrel. A BB that reaches full acceleration earlier in the barrel will actually slow down as there is no force behind it anymore, so it is traveling on inertia alone and frictional forces, due to air and also the walls of the barrel, act to slow it down. This is relevant because with high-powered sniper rifles, testing is done with a .2g BB. This is faulty because the gun is tuned to shoot with a heavier BB, like .34g or greater, let's say. Thus, that lighter BB may exit the barrel before the full energy potential is reached. For instance, there have been cases where somebody will chrono at 490fps with a .2g BB which equates to 2.2J of energy. Then, they chrono with a .36g BB at 420fps which equates to 2.9J. Same gun, very different muzzle energy because of what I outlined above. That is where muzzle energy comes into play and is a better indicator of a gun's energy capability than pure velocity. You can see how that could come into play at the chrono pit, and a player innocently and unknowingly is wielding a gun that is shooting at higher muzzle energy than expected.

This is usually only seen in higher-powered guns, but can be seen in our regular AEGs as well to a smaller degree. For example, let's say you have a gun chrono at 400fps with .2g BB, which equates to 1.48J. I have seen multiple common instances where that same gun will then chrono at 360-370fps with .25g BB, which equates to 1.49-1.58J. See how the energy rise was slight, although the gun legally shot 400fps with .2g BB?

Thus, the relationship is not linear, but is dependant on parts in your gun, including the nozzle, tightbore barrel, piston head, cylinder, cylinder head and inner barrel length. Furthermore, if those parts aren’t sealing well, that throws a bit more variance into the mix. However, due to experiences with various guns, you can get a feel for where a gun should lie in the muzzle velocity range based on what BB is being used. So your statement about expecting a gun to shoot anywhere from 25-50fps higher with a .2g BB versus a .25g BB, as a loose general number, is accurate, although in my general experience I would typically say 30-40fps is a more likely number. I threw out 25-50fps earlier to incorporate more variables.

The same discussion mentioned above for sniper rifles and regular AEGs also applies to support weapons. For example, one of my RPKs chrono'd at 442-447fps consistently at the last Roanoke op with .2g BB, and prior to going to that op, I had checked it at home with .25g BB to be anywhere from 401-411fps. The .2g BB energy works out to 1.79-1.87J, whereas the .25g BB works out to 1.85-1.94J, so you see how the same gun can produce slight muzzle energy differences just because of the BB.

This ALSO is further complicated by the brand of BBs you use. Some brands are nominally smaller than other brands. For example, I have some KSC .25g BBs that a handful measured in the 5.92-5.96mm diameter range. Compare that to some AE .25g BBs that I measured that fell in at 5.94-5.97mm range. The AE had a smaller tolerance range and a slightly larger diameter, making them more consistent and accurate. This is more apparent in gas guns, in which the hopup systems differ than that of the AEGs, and I am still learning about that having picked up types of those guns in the past couple months.

Ok, I think the only thing I haven't covered is bore-up cylinders.......they have a slightly larger internal diameter for the purpose of being used with longer inner barrels with higher-powered springs. You would not use a bore-up in a short inner barrel gun for the reasons outlined earlier…..the BB would exit the barrel before full acceleration is achieved. I personally am not a big fan of them since they require a matching piston head, and those are not readily available to my knowledge, unless you buy the entire kit again. Therefore, I have less experience with these, but the same basics apply. The thought is that if you have a larger air volume in the cylinder as the source, along with a long inner barrel, that you can achieve greater muzzle velocity since you are allowing the BB to accelerate longer with a larger pressure behind it.

Basically overall, you would pick what inner barrel length you are seeking first, and then pick a cylinder type to match it.